The vertical Francis turbines developed by Sulzer - Escher Wyss in
Zurich using model tests have a runner diameter of 7,900 mm and a
maximum runner throat diameter of 7,150 mm. They will be the largest
units in Asia (440 MW at 117.35 m net head). The runners, made from
stainless steel (13% Cr, 41% Ni), are to be manufactured by DBS-Escher
Wyss in Canada and each shipped to Pakistan in one piece. The turbines
have to operate over a very large head range (49.4 to 135.6 m net head
range). At the rated net head of 117.35 m, each turbines produces 440 MW
at an efficiency of at least 91%. The best efficiency is obtained at
82.3 m net head (>94.9%). The following table summarises the most
important characteristics of the turbines.

At heads higher than the rated values, the values of pressure and
flow permissible for tunnel 3 limit the output per unit to 440 MW. The
servomotors of the units are designed so that the flow is the maximum
425 m3/s per unit at the rated head. If all four units are not operating
at the same flow, then under operating head upto 135.6 m (maximum water
level in reservoir), the flow per unit could be increased above 425
m3/s, but not for all four units. Digital governers of type DTL 725 from
Sulzer - Eacher Wyss control the speed by adjusting the guidevanes,
subject to the pressure constraints applying for the penstock.

The generators have a unit rated output of 480 MVA and a rated
voltage of 18 kV. The turbine output will be limited by the generator to
432 MW (power factor 0.9). At rated conditions, the generator efficiency
is 98.72%. The rated speed is 90.9 min1 and the runaway speed 199 min1.
The generators are of standard design, but with very large dimensions.
They are being manufactured in Germany by a consortium made up of
Siemens and ABB (led by Siemens), and will be assembled on site. The
stator bore diameter is 14 m, the rotors weigh 1475 t and the stators
993 t.

The special layout of the hydropower station extension requires the
installation of butterfly valves of 7.35 m nominal internal diameter
upstream of each unit, as well as four relief valves with their guard
valves to limit the overpressure in the penstock.

Electricity for the

Whole Country

Thirteen single-phase transformers installed outdoors, each of 160
MVA rating, are to be supplied by Ansaldo (Italy). Hence three
transformers per unit plus one spare are available. The present
switchyard, which is already one of the largest in Pakistan, is being
extended by three new bays. The new switchyard will have SF6 type
breakers. AEG (Germany) is to supply and install this equipment.

Moreover, a new 500 kV line is to be constructed, since the
existing 500 kV lines only have the capacity to transmit the power from
two of the new units. The line is to go from Tarbela to Lahore and from
there via Multan to Jamshoro. This section is under construction and is
scheduled for completion in 1992. In the future, it is planned also to
build lines from Lahore via Faisalabad to Jamshoro and from there to
Karachi. Hence Tarbela is to be connected to Karachi, the largest city
in Pakistan, via a 500 kV line.

At present, a new load dispatch control centre is under
construction in Islamabad, the capital. This will control distribution
for the whole country. The centre is to be responsible for frequently
control; however, until it is commissioned, Tarbela is responsible for
said frequency control.

Plant Monitoring System

with Many Functions

On completion the new plant will be controlled at first from the
control room of the existing power plant. One year after commissioning
of the last unit, it is intended to introduce the SCADA remote control
and data compilation system. This will be connected to the new national
load distribution centre. The plant monitored system (PMS) to be
supplied by Sulzer - Escher Wyss, Zurich, monitors the penstock and
tunnel protection, controls the units and maximises the efficiency of
each unit. The transient parameters are measured, processed and
monitored; in addition, trend analyses are carried out.

The addition of turbine units 11 to 14 at the Tarbela Dam is the
third major extension to this project, and will increase the installed
capacity at the power station to 3510 MW. The DBS Escher Wyss contract
is to design, supply, erect and commission the 4 large Francis turbines
and 4 governor systems. The turbine runner is made from type 13.4
stainless steel and will be shop fabricated and shipped in one piece
weighing approximately 200 tonnes. The runner throat diameter is 7510
mm.

The notice to proceed was effective in March 1988, and turbine Unit
11 was scheduled to be in service in July 1991 and the contract, will
conclude with Unit 14 in service May 1992. The successful performance of
Units 9 to 10 (178 MW each) supplied DBS in 1984 was an important factor
in obtaining this contract from WAPDA.

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